Integrated circuits are widely used in many electronic applications. Current sources for these integrated circuits are often made by using active devices both as biasing elements and as load devices for amplifier stages. The use of current sources in biasing, for example, can result in improved sensitivity of circuit performance to power-supply variations and to temperature. Current sources also are often more economical than resistors in terms of the die area needed to provide bias current of selected values. When current sources are used as load devices in transistor amplifiers, the high incremental resistance of the current source can result in high voltage gain at low power-supply voltages.
Current sources, for example, can be formed from or include a field-effect transistor and a reference voltage source that biases the gate of the transistor. The reference source voltage can be a so-called "bandgap" type which generally refers to the energy interval between the valence bands and the conduction bands of a semiconductor. Current sources of this type conventionally use a known relationship of dependency between this interval and the temperature to achieve compensations that make the reference voltage as stable as possible as a function of the temperature.
A bandgap-type voltage source generally has two diodes through which flow different currents and a looped differential amplifier amplifying the voltage difference at the terminals and supplying the diodes with current. The current through the diodes can also be the same current, but if the current is the same, then the diodes are necessary and will generally have different junction surfaces. A current source can be made from this type of voltage source, but the stability in temperature is often lost during the voltage-to-current conversion.
Another type of current source is the "Wilson mirror" sources. A source of this kind is generally based upon mutually compensating variations in the characteristics of several transistors which mutually copy one another's currents. A Wilson mirror source conventionally has two parallel branches with two transistors each, and the transistors are mounted so that each branch copies the current of the other one. Two transistors which each belong to a different branch are different in size or in threshold voltage.
Modifications to this Wilson mirror type have been developed which further attempt to stabilize the current source for temperature variations. These modifications, for example, can include adding additional transistors such as in an MOSFET cascade current source illustrated in FIG. 1 or combining the Wilson mirror type and the bandgap type sources such as seen in U.S. Pat. No. 5,103,159 by Breugnot et al. titled "Current Source With Low Temperature Coefficient." Although some stability can be obtained with these modified current sources, either the stability or the performance can be considered lacking for many low power applications such as low power clocks, low current oscillators, or controlling gain stages, dividers, level shifters, or other functions of various low power integrated circuit designs.